The present invention relates to a modulation circuit and particularly to a modulation circuit used in a digital modulation system for a wireless installation.
The conventional frequency modulation circuit has a band restriction filter and a modulator. Modulation data are send to the modulation circuit. The received modulation data are input into the band restriction filter. A band of the modulation data is restricted by the band restriction filter. The band restriction data are input to the modulator directly.
Amplitude of the band restriction data is changed according to a change of an ambient temperature and a power supply voltage. Further the modulator influences a modulation factor of the modulation circuit according to the ambient temperature change. Therefore, the conventional frequency modulation circuit would not keep a constant modulation factor.
The modulation circuit of the present invention may keep a constant modulation factor regardless of the change of the ambient temperature and the power supply voltage.
A modulation circuit of the present invention includes a band restriction filter, an attenuator, a modulator, a reference voltage generator, a comparing reference voltage generator, a voltage comparator and an attenuator controller. The band restriction filter receives modulation data. The attenuator is connected to the band restriction filter for adjusting its attenuation in response to an attenuator control signal. The reference voltage generator is connected to the attenuator for providing a reference voltage independent of a temperature and a power supply voltage to the attenuator. The comparing reference voltage generator is connected to the reference voltage generator for providing a comparing reference voltage independent of the temperature and the power supply voltage. The modulator is connected to the attenuator. The voltage comparator is connected to the attenuator and the comparing reference voltage generator for comparing the comparing reference voltage with a voltage of an output of the attenuator and generating a comparison signal based on the comparison. The attenuator controller is connected to the attenuator and the voltage comparator for generating the attenuator control signal in response to the comparison signal.